1. Field of the Invention
The present invention relates, generally to methods for producing hydrogen from organic waste and more particularly to a new hydrogen fermentation batch process capable of replacing the existing complex continuous waste treatment system by using the pretreated organic waste not only as a substrate but also an inoculum.
2. Background Art
Distribution issues and limited supplies of fossil fuels have caused political and diplomatic problems as well as economic problems due to the rise of oil prices. In addition, CO2, NOx and SOx emission from burning fossil fuel is now accelerating global warming.
Research for developing alternative energy has increased and is essential for the development of the next generation, especially for countries like Korea that are highly dependent on imported fossil fuels.
Hydrogen is recognized as a remarkable energy source because it generates only water under combustion and therefore displays environment-friendly characteristics. In addition, it has the highest energy content per unit weight of any known fuel and can easily be converted to electric energy via fuel cell technology (see Momirlan, M. and Veziroglu, T. N. Renew. Sust. Energ. Rev., vol. 6, 2002, p. 141-179).
Currently, more than 90% of hydrogen is produced by the water reforming method, however this method uses fossil fuels and is intensive in energy consumption. On the other hand, the method of biological hydrogen production has not been well researched but is recognized as a latent technology in light of the fact that the material costs and energy consumption can be minimized.
The technology of producing hydrogen through anaerobic fermentation is estimated to be practicable technology in comparison to other biological methods because of advantages such as high hydrogen production rate, no use of light energy, and direct use of organic waste for substrate (see Hawkes et al., Int. J. Hydrogen Energ., vol. 27, 2002, p. 1339-1347).
In 2004, food waste accounted for 32.6% of the entire municipal waste in Korea causing a plurality of leachate and stench due to the high portion of volatile solids (85% to 90%) and water content (75% to 85%), thereby raising many problems in the treatment process. Food waste has mainly been treated by landfill, incineration and recycling but the recycling ratio is increasing because landfill has been prohibited since 2005. However, the optimum recycling plan is not yet established.
The majority of sewage sludge is an organic sludge with more than 40% being of organic content. Its production reaches 2.3 million tons every year and 70% of the sewage sludge is dumped to the sea and the rest are treated by incineration, recycling and landfill (based on figures from 2004). However, as regulations for emitting the waste to the sea were strengthened with the action of the London Convention (“96 agreement”), a method to treat sewage sludge on land is urgent.
As such, an anaerobic fermentation technology which degrades organic waste and simultaneously recovers hydrogen, itself a clean energy source, can provide a new alternative. Especially, as it is reported that when food waste is mixed with the sewage sludge to be digested, relatively deficient nutrients in the food waste are provided from the sewage sludge so that the digestion efficiency and the biogas production rate are increased (see Kim et al., Int. J. Hydrogen Energ., vol. 9. 29, 2004, p. 1607-1616).
Much research with regard to hydrogen fermentation of organic waste is in progress. Substrate pretreatments such as heat, acid, and alkali treatment improved hydrogen production by killing the non-hydrogen producing bacteria present in the substrate and increasing hydrolysis. Various kinds of waste were tested for the hydrogen production and technologies for continuous hydrogen production via process control were developed.
However, even if a pretreated substrate was used, when hydrogen is produced from the real waste in continuous operation, limitations were reported, for example, a long start-up period and hydraulic retention time were required and the hydrogen production sometimes fluctuated unstably (see Kim et al., Int. J. Hydrogen Energ., Vol. 27, 2004, p. 1607-1616; Shin and Youn, Biodegradation, Vol. 16, 2005, p. 33-44).
The major reason to improve the performance by controlling process configuration like sequencing batch reactor (SBR) or up flow anaerobic sludge blanket (UASB) in the existing anaerobic digestion process is to maintain the high concentration of microorganisms with good activity and obtain biogas from the organic components in a short time. It is especially effective when methane producing microorganisms are used which have a very slow growth rate.
However, the microorganisms with rapid growth rates like hydrogen producing bacteria showed low hydrogen conversion efficiency at long retention time by competing with the non-hydrogen producing bacteria and an unstable yield was observed in continuous operation. On the contrary, if higher hydrogen yield could be obtained by batch operation, the advantages of the continuous operation would be greatly diminished for hydrogen fermentation. In hydrogen fermentation using food waste as a substrate, the batch operation showed higher hydrogen yield in comparison with continuous operation. Although batch operation showed a lag period of 5 to 20 hours, hydrogen fermentation completed within 1 to 5 days. In case of continuous operation, the optimum hydraulic retention time was about 1.5 to 6 days, which did not show increased efficiency over batch operation (see Lay et al., Wat. Res., vol. 33(11), 1999, p. 2579-2586; Okamoto et al., Wat. Sci. Technol., vol. 41(3), 2000, p. 25-32).
Nevertheless, the batch process has a critical defect, as it is necessary to prepare an inoculum each time, hence the process is cumbersome and a pollutant addition itself.